Command circuit



y 1962 D. s. WILLARD 3,045,149

COMMAND CIRCUIT Filed July 15, 1960 THERMAL SWITCH INVENTOR. DAVID S.WIL ARD 14 ATTORNEY United States Patent 3,045,149 COMMAND CIRCUIT DavidS. Willard,'P.0. Box 5, High Rolls- Mountain Park, N. Mex. Filed July15, 1960, Ser. No. 43,254 2 Claims. (Cl. 317-147) (Granted under Title35, US. Code (1952), sec. 266) The invention described herein may bemanufactured and used by or for the United States Government forgovernmental purposes without payment to me of any royalty thereon.

The purpose of this invention is to provide a command circuit that willrespond only to a command signal meeting the following conditions: (1)An alternating signal of predetermined frequency pulsed on and offperiodically; (2) A ratio of on time to off time exceeding apredetermined value; (3) An on time less than a predetermined amount;(4) An ofli time less than a predetermined amount; and (5) An off timegreater than a predetermined amount. The necessity for satisfying all ofthese conditions insures a high degree of protection against falseoperation of the command circuit.

The invention will be explained in detail with reference to the specificembodiment thereof shown in the accomp anying drawing in which FIG. 1shows the general form of the command signal, and

FIG. 2 is a schematic diagram of a command circuit in accordance withthe invention.

Referring to the drawing, when an alternating current command signal ofthe type shown in FIG. 1, and meeting the five requirements definedabove, is applied to terminal 1, switch 2 is actuated to its closedposition. This switch may be included in any external circuit which itis desired to control by the command signal.

The circuit of FIG. 2 is designed for use with command signals in theaudio frequency range. Relay K, may be of the vibrating reed type havingan actuating coil 3, a stationary contact 4 and a movable contact 5 inthe form of a vibrating reed. An alternating current in coil 3 causesthe reed to vibrate. The amplitude of the vibration, however, will besutficient for reed 5 to touch contact 4 only when the frequency of theenergizing current equals, or very nearly equals, the resonant frequencyof the reed. Consequently, a command signal, having substantially thesame frequency as the natural frequency of the reed, will cause contacts4-5 to make and break at an audio rate, resulting in a pulsed directcurrent through the coil of.K of sufficient average value to actuatethis relay.

Closing the contacts of K connects C which has previously chargedthrough R to the potential of terminal 6, across the coil'of K allowingthis condenser to discharge through the coil and actuate K The charge inC is only enough for a momentary actuation of the relay and theresistance of R is high enough that the current flow to the coil of K isbelow that required to hold the relay in the energized state. However,when K is energized by C a holding circuit is established from the coilof K through the contacts of K and contacts 1-2 of K to condenser C Thiscondenser is larger than C, and was previously charged through contacts1--2 to the voltage of terminal 6. The discharge of C through the coilof K holds this relay energized either until K opens or until thedischarge current has fallen to the dropout level of the relay. It istherefore seen that K can not remain energized for longer than a fixedinterval determined principally by the size of C The R C circuit isdesigned to have a very short time constant so that C recharges almostimmediately upon release of K and closure of contacts 3-2. For

3,045,149 Patented July 17, 1962 ICC? example, R may have a resistanceof 47 ohms and C; a capacity of microfarads, giving a time constant ofabout 0.5 millisecond.

The R C circuit is designed to have a relatively long time constant ascompared to the R --C circuit. For example, R may have a value of 68000ohms and C a capacity of 0.5 microfarad, giving a time constant of 34milliseconds. Since C can not charge while the contacts of K are closed,the time constant of this circuit determines the minimum ofi period ofthe command signal. It the command signal has an o period less than thisminimum, including a zero off period (continuous signal), only a singleactuation of K can occur.

During the time that K is actuated, heating element 7 of thermal switch2 is energized through contacts 45.

. This switch is designed so that, when pulses of current of a constantspecified amplitude are periodically applied to it, the ratio of thepulse duration to the interval between pulses, i.e., the ratio of the ontime of the energization to the olf time, must exceed a predeterminedvalue to produce enough heating to cause the switch contacts to close.For example, the predetermined value of this ratio may be unity. Statedin other words, the average value of the pulsed current flow in element7 must exceed a predetermined amount in order to close the switchcontacts.

Returning to the five previously listed conditions that must be met bythe command signal before it is capable of causing actuation of switch2, the features of the circuit responsible for these requirements are asfollows:

Condition 1: The signal must be an alternating signal of the requiredfrequency to actuate relay K The signal must be pulsed in order tosecure energization of thermal switch 2 over a suificient period of timeto cause its contacts to close. If the signal were continuous, thecontacts of K would remain closed and only a single actuation of Kapplying only a single pulse of energy to switch 2, would occur.

Condition 2: Since, for switch 2, the ratio of the heating current pulseduration to the interval between current pulses must exceed apredetermined value to operate the switch and since the characteristicsof the K circuit prevent this ratio from ever exceeding the on time tootf time ratio of the command signal, it follows that the latter ratiomust exceed the predetermined ratio for the switch 2 in order to effectoperation of the switch.

Condition 3: As already explained, relay K will not remain energizedlonger than a fixed interval determined mainly by the size of C Themaximum duration of energizing pulse for switch 2 equals this fixedinterval. For this maximum pulse duration, there is a correspondingmaximum interval between pulses above which the required energization ofswitch 2 will not be achieved. If the on period of the command signalexceeds the K fixed interval plus the above defined maximum intervalbetween pulses, switch 2 will not be actuated.

Condition 4: If the command signal off time exceeds the above definedmaximum interval between pulses, actuation of switch 2 will not occur.

Condition 5: If the command signal off time is less than the timerequired for C to receive sufiicient charge to actuate K only a singleactuation of K will occur and, consequently, switch 2 will not beactuated.

I claim:

1. A command circuit comprising: frequency sensitive means, having a setof normally open contacts, for receiving a command signal and closingsaid contacts when said signal has a predetermined frequency; a relayhaving two sets of normally open contacts, a set of normally closedcontacts and an actuating coil therefor; first and second capacitors; asource of direct current; means including a resistor for connecting saidfirst capacitor across a said source; means including the contacts ofsaid frequency sensitive means for connecting the first capacitor acrossthe coil of said relay; means including the normally closed contacts ofsaid relay for connecting the second capacitor across said source; meansincluding one set of normally open contacts of said relay and thecontacts of said frequency sensitive means in series for connecting thesecond capacitor across the coil of said relay; a current actuatedswitch responsive only to an energizing current having an average valuein excess of a predetermined amount; and means for applying anenergizing (5. current to said switch through the other set of normallyopen contacts of said relay.

2. Apparatus as claimed in claim 1 in which said switch is a thermalswitch having a heating element through which said energizing currentpasses.

References Cited in the file of this patent UNITED STATES PATENTS2,347,194 Holliday Apr. 25, 1944 2,724,074 Welker Nov. 15, 19552,914,709 Rabinow Nov. 24, 1959

